The invention relates to a process for measuring and controlling the nip pressure in the press of a paper machine, particularly in a shoe press, across and/or along the web running direction. In addition, the invention relates to a device for carrying out the process.
In order to control dewatering by presses used in the paper-making sector, the appropriate nip pressure must be set. That is why there are a number of theoretical calculation approaches. These can be applied as long as conditions are ideal. Particularly in the edge zones of a paper web however, for example at the creping cylinder of a tissue machine, non-uniform conditions occur that can no longer be expressed precisely in a calculation. On the other hand, there are also operating parameters that can be calculated, but production realities prevent them from being entered as actual settings.
The aim of the invention is thus to create a process and a device to measure and control the nip pressure across and/or along a press in order to obtain an even moisture profile or to set paper quality parameters.
The invention is characterised by the hydraulic, static pressure in the press gap (press nip) being measured at reference points through measurement boreholes exposed to the nip. By measuring at reference points, it is very easy to determine the nominal pressure value for controlling the nip pressure in the cross direction and the actual pressure curve in running direction.
An advantageous further development of the invention is characterised by the nip pressure in the press being controlled at this nominal value in cross direction and thus, also being adjusted continuously. With this control method it is possible to obtain uniform press duty across the machine running direction at any production status.
A favourable configuration of the invention is characterised by differential set values for the pressure across the web width. As a result, the moisture profile of the paper web can be influenced selectively.
The invention is also directed to a device for measuring and controlling the nip pressure in the press of a paper machine, particularly in a shoe press, across and/or along the web running direction. It is characterised by measurement boreholes being provided at several points in the press across and/or in web running direction, and which extend into the press nip. By measuring the pressure at the reference points, it is possible to determine the entire pressure progression exactly and control it accordingly.
If measurement boreholes are provided in the edge zone, the areas that cannot be calculated can be controlled particularly well.
If several pressing elements, for example pressing pistons, are provided across the web width and which are of a design that allows them to be controlled partly or entirely, the nip pressure profile or also the moisture profile across the web running direction can be controlled particularly well.
Due to the adjustable shoe limit stop, the position of the press shoe can be moved in longitudinal direction and the longitudinal profile of the nip pressure in the nip thus set and/or controlled.